This invention relates to a process for producing a 2-tertiary-alkyl substituted anthraquinone, and particularly relates to a process for producing a 2-tertiary-alkyl substituted anthraquinone, characterized by carrying out catalytic oxidation of a diphenylmethane type compound having the formula ##STR2## wherein R.sub.1 and R.sub.2 are different from each other, and independently hydrogen or tertiary-alkyl having 4 and 5 carbon atoms with the proviso that one of R.sub.1 and R.sub.2 is hydrogen, and R.sub.3 is alkyl having 1-3 carbon atoms, in vapor phase in the presence of a catalyst comprising vanadium oxide.
In the prior art, a process for producing an alkyl substituted anthraquinone which comprises reacting phthalic anhydride with an alkyl benzene in the presence of a Friedel-Crafts catalyst was known. In this process, the operation is complicated. In addition, since a large amount of aluminum chloride and a large amount of concentrated sulfuric acid are necessary in this process, disposal of the wastes from the reaction is difficult. Furthermore, since isomerization reaction, rearrangement reaction and elimination reaction of the alkyl group occur in the above process, many by-products derived from these reactions contaminate the object product.
The processes for producing a non-substituted anthraquinone are disclosed in Japanese Publication (first) Nos. 47-576; 48-61463 and 49-75568. These processes comprise carrying out catalytic oxidation of indanes, such as 1-methyl-3-phenyl indane, or diphenylmethanes, such as o-benzyltoluene, in vapor phase. However, we have found that when catalytic oxidation of o-benzyltoluene was carried out in vapor phase, a large amount of p-benzylbenzaldehyde was formed as a by-product, and therefore, the yield of anthraquinone was as low as from 40 to 45%. We have also found that when catalytic oxidation of o-benzyltoluene substituted by an alkyl group, such as methyl, ethyl or propyl, was carried out in vapor phase, the oxidation of the alkyl group substituted on the benzene nucleus occurred, and therefore, the yield of alkyl substituted anthraquinone was decreased.